A BIRTHDAY party organised by two Sydney brothers turned ugly after its location was posted on Facebook and hundreds of drunken teenagers crashed it, police said.

The Public Order and Riot Squad had to be called to the 18th birthday party at Winston Hills on Saturday night after more than 300 teenagers arrived. Five people were arrested after some of the mob attacked a policeman and damaged the officer's vehicle.

Police called back-up to help close down the party in Caroline Chisholm Drive at about 11pm.

"Police received a noise complaint and attended the scene where two brothers aged 18 and 21 were having a party," a police spokesman said yesterday.

"When they arrived police were confronted with a large number of intoxicated people, mainly teenagers in the house and on the premises. There was also loud music playing.

"The property owner and police closed the party down, and further police including the Public Order and Riot Squad were called to help disperse the crowd."

Police said the party numbers grew way above what was expected because the address was posted on Facebook.

"Parents and teenagers should keep all parties on a locked profile," he said.

A police vehicle windscreen was smashed and five people were arrested.

Police patrolling nearby Romulus St were confronted by an intoxicated 16-year-old from Baulkham Hills who was one of the partygoers. He was arrested after allegedly assaulting an officer.

When police returned to their vehicle they found the windscreen had been smashed with a block of wood. Police also arrested an intoxicated 23-year-old Wentworthville man after he allegedly smashed the window of a house. p

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